In many downhole drilling and measurement systems, a downhole power source is required. The power source can include direct power output from the torque and rotation of the drill string, electrical storage batteries, and turbines, among others. In a drilling environment where mud flow is present, there is an opportunity to use part of this hydraulic power to drive a turbine. The turbine can, in turn, rotate a variety of electrical, mechanical, or other devices to convert the hydraulic energy into a desired power output.
Turbines, although efficient, must be operated within a narrow rotational speed range for optimum power output. The rotational speed of the turbine is related to the flow rate or velocity of the drilling mud. It is desirable to extend or maximize the range of flow rates (minimum to maximum) over which optimum power output can be achieved, such that the downhole operation can be used with the broadest possible hydraulic parameters desired in the drilling process.
Various techniques have been developed for manipulating flow through a turbine, such as U.S. Pat. No. 6,402,465, issued to Maier. The Maier patent provides a ring valve for turbine flow control for industrial turbines with compressible flow. In this case, the overall mass flow focuses control on the apparatus and fails to disclose the use of an incompressible flow, velocity approach. There are various other downhole systems, such as measurement while drilling (MWD) tools, turbodrills, etc., that use turbines for power generation. However, so far as known to applicants, these devices fail to provide techniques capable of extending flow ranges.